using index in execution plan

HI all,

I have a table named RTBS_SUBS_EVENT and there is a date column named expiry_date.

expiry_date is a partitioned column too. a column named ID and it's indexed.

NLS_DATE_FORMAT of the database is 'DD-MON-RR'.

I run those queries and they return the different explain plan:

Query 01:

SELECT *

FROM stage.RTBS_SUBS_EVENT 

WHERE event_date > '25-Sep-2018'

AND subscriber_ID = '84523891696';

Plan hash value: 2001189060

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| Id  | Operation                | Name            | Rows  | Bytes | Cost (%CPU)| Time     | Pstart| Pstop |

------------------------------------------------------------------------------------------------------------

|   0 | SELECT STATEMENT         |                 |     1 |   140 |    28   (0)| 00:00:01 |       |       |

|   1 |  PARTITION RANGE ITERATOR|                 |     1 |   140 |    28   (0)| 00:00:01 |  1729 |  1826 |

|*  2 |   TABLE ACCESS FULL      | RTBS_SUBS_EVENT |     1 |   140 |    28   (0)| 00:00:01 |  1729 |  1826 |

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Predicate Information (identified by operation id):